1. Field of the Invention
The present invention relates to an optical disk that is used for information recording or reproduction.
2. Related Background Art
It has been desired in the area of optical disks to achieve higher-density information recording. A method for realizing such high-density recording has been proposed in the form of a DWDD (domain wall displacement detection) type optical disk.
In the DWDD type optical disk, it is necessary to weaken magnetic coupling between adjacent recording tracks. Therefore, when manufacturing the DWDD type optical disk, prior to recording of information signals, a magnetically separating process for magnetically separating adjacent recording tracks is performed. The magnetically separating process is performed by methods such as those disclosed in JP 6(1994)-290496 A and JP10(1998)-340493 A.
A structure of the DWDD type optical disk and a method of magnetically separating process according to a conventional technique are described as examples with reference to FIG. 5. In an optical disk shown in FIG. 5, a first dielectric layer 502, a magnetic layer 503, a second dielectric layer 504, and a protective coating layer 505 are laminated in this order on a substrate 501. On a surface of the substrate 501 on a side of the thin film layers, grooves 506 are formed. A portion between two of the grooves 506 adjacent to each other in a radial direction is referred to as a land 507 that is used as a recording/reproducing track. The grooves 506 have a width of, for example, 0.2 xcexcm, and the lands 507 have a width of 1.4 xcexcm. The magnetic layer 503 includes at least three magnetic thin films that are used for reproduction employing a DWDD system. In order for the DWDD system to be performed, it is necessary to magnetically separate the lands 507 as the tracks used for recording/reproduction.
The following description is directed to a method of performing the magnetically separating process with respect to this optical disk. In the magnetically separating process, a laser beam 508 to be used for annealing is focused on the groove 506 by an objective lens 509 and allowed to scan along the grooves 506, so that magnetic coupling between the magnetic layers 503 on the grooves 506 is lost. As a result, in each of the lands 507, which is a region interposed between the grooves 506 that have been subjected to annealing, both sides of the land 507 is magnetically separated, thereby allowing a DWDD operation to be performed. The laser beam 508 used in this process has, for example, a laser power of 2 mW and a wavelength xcex of 780 nm. The objective lens 509 has a NA of 0.5, and a beam spot of about 800 nm in diameter is formed. The beam spot of the laser beam 508 travels at a speed of, for example, 2 m/second.
When the lands 507 interposed between the grooves 506 are used as the recording/reproducing tracks as in the foregoing description, at an innermost or outermost end of a recording/reproducing track region, the groove 506 is provided. That is, even the land 507 positioned at the innermost or outermost end is interposed between the grooves 506. Accordingly, both sides of each of all land tracks can be magnetically separated by allowing the laser beam 508 for annealing to scan over all the grooves 506.
In many of recordable type optical disks with tracking guide grooves, an address represented by a track number and a sector number is composed of pit rows of a length of several tens of microns. In regions in which the pit rows are provided, the grooves are interrupted. That is, these pits are provided in flat surfaces without the grooves.
Essentially, an optical disk is designed so that a light beam is incident in a direction perpendicular to a disk surface, whether in recording or in reproduction. When a light beam is incident at an angle (when a tilt is caused), it causes adverse effects such as deterioration in recording sensitivity, reproducing sensitivity, and resolving power, crosstalk, crosserase, or the like.
On the other hand, as the information oriented society advances, optical disks have been requested to achieve higher-density recording, and thus demands for adaptability to a high-NA objective lens and reduction in thickness of a disk substrate have been growing. Further, as wider applications of optical disks are being found, environments in which optical disks are used in portable devices such as a still camera and a video camera have been increasing. These environments, however, cause more tilting between a light beam and a disk, and thus some tilt correction methods have been proposed.
The simplest and most commonly used among such methods is a method shown in FIG. 6. The method utilizes a phenomenon in which an intensity distribution of reflected light depends on a tilt. A laser beam 508 that is used for recording/reproduction is focused by an objective lens 509 via a beam splitter 601 and irradiated onto an optical disk 501. However, when the optical disk 501 is not perpendicular to the laser beam 508, namely, a tilt is caused as shown in the figure, a portion of a reflected light beam 602 reflected from the optical disk 501 is not incident on the objective lens 509. As a result, a differential output corresponding to the tilt is detected from a differential amplifier 604 of a bisected detector 603.
However, in this method, it is necessary as a precondition that no diffraction or variations in reflection should be caused in a beam spot when the reflected light beam 602 is reflected from the optical disk 501. Thus, tilt detection is performed using flat portions (hereinafter, referred to as mirror portions) without grooves on the optical disk 501.
As shown in FIG. 5, when tracks are magnetically separated by allowing a light beam to scan, in each magnetic film positioned between the tracks, magnetic anisotropy is reduced, and a physical and structural change is caused in a central portion. This causes the complex refractive index to be changed, and as a result, reflectance is reduced. Therefore, in a disk having this configuration, when a recording/reproducing beam passes over a track, the reflectance of portions on both sides of the track changes, and thus employing discontinuous grooves and simply providing the mirror portions are not sufficient to enable exact detection of the tilt.
It is an object of the present invention to provide an optical disk in which, while adjacent recording/reproducing tracks are magnetically separated so that a DWDD operation can be performed, tilting can be detected based on an intensity distribution of reflected light.
An optical disk of the present invention includes a disk-shaped substrate and at least a first dielectric layer, a magnetic layer, and a second dielectric layer that are formed on the substrate. In the substrate, a predetermined region ranging in a radial direction is used as a data region for recording/reproducing data. The data region includes recording/reproducing tracks that are composed of a plurality of discontinuous lands or grooves ranging from an innermost track to an outermost track. The magnetic anisotropy of each of the magnetic layers positioned between the respective recording/reproducing tracks is reduced to a level lower than that of the magnetic anisotropy of the magnetic layers positioned on the recording/reproducing tracks, so that the magnetic layers are magnetically separated only between the recording/reproducing tracks respectively and not magnetically separated in flat portions in which the grooves are interrupted.
According to this configuration, since the magnetically separating region is not provided in the flat portions (mirror portions) in which the recording/reproducing tracks are interrupted, the reflectance is not changed over the flat portions. Accordingly, based on a reflectance held so as to be even, in recording or reproduction, a tilt of a, disk can be detected using light reflected from the mirror portions. Thereby, a change in an optical system caused by the tilt is corrected, and thus an optimum recording power or an optimum reproducing power can be maintained. This process of correcting a tilt is effective particularly in an optical disk employing a DWDD system in which in reproduction, a domain wall is displaced using a temperature distribution and detected.
An optical disk of another configuration according to the present invention includes a disk-shaped substrate with pits and grooves formed by emboss processing, and at least a first dielectric layer, a magnetic layer, and a second dielectric layer that are formed on the substrate. In the substrate, a predetermined region ranging in a radial direction is used as a data region for recording/reproducing data. The data region includes a plurality of recording/reproducing tracks ranging from an innermost track to an outermost track, and is divided into a plurality of segments in a tangential direction. Each segment includes a pit region and a groove region. The pit region is provided with at least a pair of wobble pits to be used for sample servo. The groove region is provided with the recording/reproducing track. The recording/reproducing tracks are composed of the grooves. The magnetic anisotropy of the magnetic layer positioned on each of lands between the respective recording/reproducing tracks is reduced to a level lower than that of the magnetic anisotropy of the magnetic layers positioned on the grooves, so that the magnetic layers are magnetically separated only between the recording/reproducing tracks respectively and not magnetically separated on extensions of the lands in the pit regions.
A method of magnetically separating tracks of an optical disk according to the present invention is a method of magnetically separating the recording/reproducing tracks when the optical disk of the aforementioned configuration is provided. The method includes a process in which a light beam converged to a degree higher than that of a light beam to be used for recording/reproduction is allowed to scan over the lands that correspond to a portion between the recording/reproducing tracks, so that the magnetic anisotropy of the magnetic layers positioned on the lands is reduced to a level lower than that of the magnetic anisotropy of the magnetic layers positioned on the grooves. In the process, when the light beam passes over the pit regions, the power level of the light beam is reduced to such a degree as not to cause irreversible change in the magnetic layers.